1. Field of the Invention
This invention relates to electroless metal plating and more particularly, to a method for operation of an electroless metal plating solution having evaporative losses of at least one percent per plating cycle.
2. Description of the Prior Art
Electroless metal deposition refers to the chemical plating of a metal such as nickel, cobalt and the like over an active surface by chemical reduction in the absence of external electric current. Processes and compositions useful therefor are described in numerous publications including, for example, U.S. Pat. Nos. 3,123,484; 3,148,072; 3,338,726; 3,719,508; 3,745,039; 3,754,939 and 3,915,717 (example 8) all included herein by reference.
Known electroless deposition solutions generally comprise at least four ingredients dissolved in a solvent, typically water. They are (1) a source of metal ions, (2) a reducing agent such as hypophosphite, an amine borane, or a borohydride, (3) an acid or hydroxide pH adjustor to provide required solution pH and (4) a complexing agent for the metal ions sufficient to prevent their precipitation from solution. Other minor additives include stabilizers, brighteners, alloying agents, surfactants and the like as is known in the art.
In general, metal deposition involves the reduction of metallic ions to metallic form by the action of a reducing agent, typically the borane, borohydride or the hypophosphite ion or the reaction product of the hypophosphite ion with water. Using hypophosphite as the reducing agent, a metal deposit is a phosphorus alloy.
The deposition or reduction reaction is initiated by contact with a catalytic surface such as a catalytic metal work-piece or a catalyzed non-conductor. Once initiated, deposition is autocatalyzed by the metal placed onto the surface of the work-piece. The deposition reaction, using a nickel-hypophosphite plating bath for illustration, can be represented by the following reaction: ##EQU1##
The above equation can be rewritten for specific reactants, using nickel sulphate and sodium hypophospite as exemplary reactants, as follows: ##EQU2##
The deposition reaction for an amine borane using dimethylamine borane and nickel chloride for purposes of illustration is set forth below: ##EQU3##
From the above equations, it should be evident that the composition of a plating solution changes continuously throughout a plating reaction. For example, in the above reaction, nickel is depleted by plate-out onto a work-piece, reducing agent is consumed by oxidation -- i.e., sodium hypophosphite is oxidized to sodium dihydrogen phosphite and possibly, some sodium hypophosphate and the anion of the nickel salt forms an acid with hydrogen liberated during the plating reaction. Thus, throughout the above plating process, nickel concentration decreases from its initial concentration, oxidation products and acid concentrations increase and pH changes as acid is formed. These compositional alterations eventually cause change in the quality and uniformity of a metal plate as well as in plating rate.
The art, well aware of the aforesaid compositional variation taking place during plating, has attempted to compensate for the same by frequent replenishment of bath constituents such as by replenishment with metal salts, reducing agents and pH adjusters. Other replenisher constituents may be added such as complexing agents, stabilizers, and the like, even though these materials are usually non-reactive. Replenishment of these materials is needed to compensate for losses due to drag-out, consumption and the like.
Replenishment is accomplished to periodic addition of either dry replenisher components or concentrated aqueous solutions thereof so that the concentration of each component is returned to substantially its initial concentration. The replenisher may be admixed prior to addition or added separately. Aqueous solutions are preferably used for replenishment as the addition of dry powders can trigger the plating bath if careful control is not exercised.
Notwithstanding the above replenishment practices, difficulties in the quality and uniformity of the metal plate, and changes in plating rate are encountered. The difficulties are, to a large extent, due to continual build-up of reaction by-products as plating proceeds. Thus, though initially zero, there is a gradual, but steady increase in the concentrations of by-products as well as salts formed by neutralizing acid formed during reaction. Though the prior art replaces depleted constituents through replenishment, no provision is made for removal of by-products continuously during use.
By-product content is not a serious problem through the first several cycles of plating (as defined hereinafter) because the concentration of by-products is initially low. However, dependent upon the substrate plated, the initial concentration of the metal ions in solution, and the pre-treatment of the substrate, by-products become troublesome as plating proceeds. For example, when plating an active substrate such as aluminum with a nickel plating solution containing about seven or more grams of nickel as metal, solution by-products are a serious problem of the third or fourth plating cycle. For less active substrates, such as catalyzed plastic or non-active metals such as mild steel, by-products are a serious problem by about the 6th to 8th cycle. As a consequence, an electroless solution is frequently dumped after from about 3 to 10 plating cycles thus requiring shutdown of the plating line for preparation of fresh solution resulting in lost time and costs known to be associated with shutdowns and disposal of used solutions.